Hydromechanical and friction retarder brakes have been employed extensively in the past for controlling vehicle speed on downhill grades. Past systems have typically relied exclusively on manual control by the operator. One such system is provided by the assignee herein on its off-highway dump trucks. These trucks are equipped with oil-cooled, pressure actuated disk brakes on all four sets of ground engaging wheels. The brakes are normally controlled by a manually operated brake pedal which is spring biased to a non-braking position in the absence of external force. A retarder control lever is provided to enable the operator to select a braking force for retarding purposes. The retarder lever remains at the position selected by the operator until it is forceably moved to a different position. The brake system is adapted to control the vehicle brakes to the greater of the two braking forces as requested by the brake pedal and the retarder control lever, thereby enabling the operator to override the retarder braking level with the brake pedal. Typically, the operator sets the retarder lever at the top of a hill. However, the braking level required to maintain a constant speed during descent of the hill depends on several factors including the slope of the hill and the vehicle load. The lever must be adjusted to maintain the desired speed during descent of the hill and, therefore, performing optimum brake control is difficult. It is possible for the operator to overheat the brakes on long descents. Hence, it is desirable to minimize operator control over the retarder brakes.
More automated brake systems have been developed to reduce the amount of operator input required to control the retarder brakes. Typically, these controls include a speed setting lever which enables the operator to select a desired ground speed. Actual ground speed is then sensed and used in a closed-loop control to regulate the brakes so as to control the vehicle speed to the desired speed. However, in such systems it is possible for the operator to overheat the brakes by selecting a high desired ground speed while operating the vehicle in a high gear. It is also possible to overspeed the engine if the operator selects a high ground speed and a low transmission gear.
In recognition of these problems, a more complex braking system has been proposed in U.S. Pat. No. 4,485,444 which issued on Nov. 27, 1984 to Maruyama et al., hereinafter referred to as Maruyama. Maruyama similarly operates on a closed loop ground speed control strategy. To prevent brake overheating, Maruyama is equipped with first temperature sensors for detecting the temperature of the cooling oil supplied to the vehicle's disc brakes. If the brakes become too hot, the controller overrides the desired speed, as set by the operator, and gradually reduces the desired ground speed in an attempt to prevent brake overheating. If the first temperature sensor were to fail or malfunction, it would be possible to overheat the brakes. In view of this, Maruyama provides a second temperature sensor for detecting faulty operation of the first temperature sensor. The system proposed by Maruyama adds unnecessary cost and complexity to the vehicle control system. Therefore, it is desirable to provide a less complex system in which temperature sensors are unnecessary and operator input is minimized.
The subject invention is directed to overcoming the above problems.